1. Field of the Invention
The invention relates to a technique for estimating in real-time friction torque in a vehicle engine whereby driveability problems due to inaccurate friction torque estimates are avoided.
2. Background Art
The performance of an engine control system depends on accuracy of an engine torque model. One of the important parts of the engine torque model is engine losses, which include pumping and friction losses.
Friction torque can be pre-calibrated and presented as a look-up table with two input variables (engine speed and indicated engine torque). Variability and changes of the engine components over time, as well as changes in the external environment, have a direct impact on engine friction torque, and hence on driveability performance. There exists a need, therefore, for development of real-time adaptation algorithms to improve accuracy of a friction torque component of the engine torque model.
One opportunity for estimating friction losses is during engine idle when the engine is decoupled from the driveline. The idle state, however, can provide an estimate of the friction losses only at idle speed and low indicated torque. All the nodes of the friction look-up table could be adapted by using new values of the friction torque at idle. However, even small errors in friction estimation at idle due to errors in accessory loads, for example, could lead to significant errors in the friction estimation at high rotational speeds. Moreover, the friction losses due to aging of engine components could also change as a function of the engine speed. Therefore, more points for different engine speeds and loads are required for successful adaption of a friction look-up table.
A more promising opportunity for obtaining relatively accurate estimates of friction torque is the period following engine start. At engine start, the engine speed increases to a relatively high level (compared with the idle speed), and then slowly decreases, converging to the desired idle speed. This period when the engine speed decreases provides an opportunity to estimate engine friction torque. This is discussed in A. Stotsky U.S. Pat. No. 7,054,738. A friction estimation technique that estimates friction during start and idle gives better results than a technique that estimates friction at idle only. However, better accuracy of the engine torque estimation can be achieved if more measurements of friction torque are available at high rotational speeds.
Friction losses consist of valve gear friction, piston ring friction, piston and connecting rod friction, and crankshaft friction. The friction losses increase with speed. Approximately two-thirds of engine friction occurs in the piston and piston ring assembly. Friction force on the piston assembly has a direct impact on piston acceleration, and hence on crankshaft speed variations. Wear and frictional changes with time of the engine components also affect friction losses and, in turn, crankshaft speed variations.
The amplitude of the crankshaft speed variations, which are induced by the periodic individual cylinder compression/expansion events, depends on compression pressure, friction force and viscosity of lubricating oil. It provides a mean for estimation of the engine friction and pump torques when the engine is not fueled. The same amplitude provides a mean for estimation of the engine brake torque when the engine is fueled.
In a fuel control for spark ignition automotive engines, fuel cut-off operation temporarily stops fuel injection. For example, fuel cut-off is activated when the throttle valve is completely closed and the engine speed is higher than a predetermined value (usually this threshold value of the engine speed is around 3000 rpm).
When it is determined that the driving state is in a decelerating operation state, no fuel supply is required, and thus a fuel cut-off is performed in order to enhance fuel consumption efficiency, purify exhaust gas and prevent heating of the exhaust gas purifying catalyst. During fuel cut-off operation, engine cylinders do not produce any torque, and the amplitude of crankshaft speed variations, which is usually a superposition of the variations induced by the combustion forces, friction and pump forces on the piston assembly, has a component that corresponds to the friction and pump forces, providing a mean for estimation of engine losses.
Since the fuel cut-off operation is usually performed at high rotational speeds (higher than 3000 rpm), this gives an opportunity to provide new measured data of the friction torque at high rotational speeds. Then the engine friction look-up table can be adapted if new data of the engine friction torque at high rotational speeds are available. However, crankshaft torsional vibrations, inertia torque due to reciprocating masses, piston mass imbalance, and other mechanically induced vibrations affect behavior of high resolution engine speed when the engine is not fueled.